* Yes, the spawn rate is 100% on play rank S and it goes down to 5% (~ rank C or D) Below these ranks, no rare enemy will spawn.

* Rare enemies will only spawn at specific positions in a stage. For a 100% spawn your play rank have to be S when you reach this position.

* The extra weapon unlocks from rare enemies are locked during the action master and action fan challenge.

Details for victory rating:

Warning: Spoiler![ Click to expand ][ Click to hide ]

* The rating depends only on boss time and number of killed enemies during the stage.

* While the boss time is a fixed value, it's a bit more complicated with the number of enemies: Due to the fact that the number of spawned enemies will vary with play rank this time, the number of necessary kills for a S rating will vary too. If you have a good boss time and your rating is still A, then you propably have missed too many enemies during the stage. Try to focus on shooting down almost every enemy.